The invention concerns a method of optimizing the design and operation of a reduction process for iron-containing charge materials, preferably in lump form, in a reduction shaft to which reduction gas is fed, for example from a fusion gasifier, with a reduced product, for example iron sponge, being taken from the reduction shaft for the production of liquid pig iron or liquid primary steel products.
The reduction shaft may be, for example, the shaft of a direct reduction process or the prereduction stage in the solid phase of a smelting reduction process. In the latter case, the charge materials, such as iron ore, preferably in the form of lumps or pellets, if appropriate with additions, in the production of liquid pig iron or liquid primary steel products are reduced in a reduction shaft directly to form iron sponge and the latter is charged into a fusion gasifying zone and smelted there while carbon carriers and oxygen-containing gas are fed in. This produces a CO- and H2-containing reduction gas, which is drawn off from the fusion gasifying zone and introduced into the reduction shaft, where it is converted and, once reduction of the iron-containing charge materials has taken place, is drawn off as top gas.
In a production process of this type, it is difficult to estimate the optimum level of production, since specific properties of the charge materials, such as stability, friability during the reduction or agglomeration, and of the reducing agents influence production.
Even today, new plants are still operated on the assumption that the raw material and reducing agents will be of high quality, which does not reflect the supply situation in the raw materials sector. Shortages in raw material supplies, and associated production stoppages are the consequence, since the limits for operation with raw materials of lower quality are not known.
On the other hand, in the design of new reduction shafts of increased or changed geometry and in the use of changed charge materials in existing plants there are uncertainties as to the effects of these changes. Uncertainties exist in particular with respect to the material flow, the dead zones of the burden and their effect on the gas flow. These uncertainties can also only be dispelled partly by experiments on actual plants or scale models. Therefore, when assessing the influence on the reduction process of the geometry of the reduction shaft and the characteristics of the raw materials, it is still necessary to rely on the experience gained from operating existing reduction plants and is very risky in particular to apply findings to previously untried geometries or charge materials, with no objective or quantitative conclusions being possible.
Already existing so-called xe2x80x9cblack boxxe2x80x9d models take into account the processes in the reduction plant or in the reduction shaft only inadequately, since these models are based on empirical relationships but cannot provide any information concerning the internal states of the reduction shaft.
The object of the present invention is thus to overcome the disadvantages mentioned by developing a method with which a reduction process can be quantitatively assessed in the entire reduction shaft and, as a result, the reduction process can be optimized.
The invention is characterized in that the reduction process is described by means of a mathematical-physical-chemical process model, in that the reduction shaft is modelled multi-dimensionally, in particular three-dimensionally, in that the process model is numerically evaluated and the results of the evaluation, obtained as multi-dimensional, in particular spatial, distributions of physical or chemical variables, are taken into account for the reduction process.
What is novel about this invention is that it allows for the first time a multi-dimensional quantitative determination of the physical and chemical variables in the entire reduction shaft and consequently objectifiable statements can be made concerning the reduction process, so that the use of this simulation tool means that there is less risk involved in the design and operation of new plants as well as the operation of existing plants with changed charge materials.
For the creation of the process model, the geometrical dimensions of the reduction shaft, the chemical and physical properties of the individual substances involved in the process, the boundary conditions necessary for solving the differential equations and the process parameters serving for controlling the reduction process are prescribed.
The result of the calculation of the process model provides for each phase at least the spatial distribution of pressure, velocity, volume fraction, chemical composition and the spatial temperature distribution in the reduction shaft.
The invention may be applied particularly advantageously to a prereduction stage, mentioned at the beginning, in the solid phase of a smelting reduction process, in that the mathematical-physical-chemical process model is created for a reduction shaft to which reduction gas is fed from a fusion gasifier, with a solid product, for example iron sponge, being introduced into the fusion gasifier from the reduction shaft.
The invention is further characterized in that the process model is created with the dust deposition and dust redispersion taken into account. As a result, the influence of the dust contained in the reduction gas on the reduction process is taken into account. This takes place for example by the dust deposition being modelled by changing the volume fraction of the dust deposited.
It is also advantageous if the process model is created with non-linear properties of the solid matter taken into account. This permits a faithful description of the flow of solid matter, in particular whenever the solid matter is modelled as a Bingham-like fluid with a yield criterion, such as a Drucker-Prager, Von Mises or Tresca yield criterion. As a result, the presence of a critical shearing stress of a granular solid substance is taken into account, so that, for example, dead zones can be calculated.
As a result of the fact that states of equilibrium are taken into account in the modelling of the chemical and physical processes, and the temperature dependence is taken into account, the process model can replicate even better the real states in the reduction shaft.
In the modelling of the chemical and physical processes, kinetic theorems are used. By using the kinetic theorems, the chemical and physical processes are modelled in the process model as they proceed over time, which permits a simulation of the spatial reaction behaviour at every location in the reduction shaft. The term kinetic means in this context that a process under consideration proceeds with a certain velocity.
A preferred embodiment of the invention provides that the substances involved in the process are assigned to individual phases, such as for example the gas phase or at least one granular phase or at least one dust phase, in the process model. A granular phase is characterized by a specific grain size and by a specific raw material. The assignment to individual phases allows every phase to be modelled according to its physical or chemical properties.
It is consequently provided that, for each phase, a mass balance of this phase and the corresponding component balances are created. These can be used to determine the volume fraction and the chemical composition of the individual phases in the reduction shaft.
The element fractions of specific chemical elements, for example in the form of mass fractions, can be calculated from the component balances. For example, for calculating the degree of metallization, the mass fractions of iron can be calculated from the component fractions, such as mass fractions, of Fe, FeO, Fe2O3 in one or more phases.
It is further provided that, for the creation of an impulse balance and an energy balance, in each case a number of phases are combined into a group, the phases of one group having the same velocity, pressure and temperature field. This may take place in the form that the gas phase and the phase of the dust dispersed in the gas are assigned to a first group of gaseous phases and the granular phases and the phase of the dust deposited in the solid matter are assigned to a second group of solid phases, a corresponding impulse balance and an energy balance being created for each of these two groups. By combining the individual phases in two groups, which are then considered as two phases, the velocity, pressure and temperature distributions can be created in a particularly simple way.
A modular treatment of the individual processes allows them initially to be considered separately, it being possible for the equations, in particular differential equations, of the individual balances to be solved by the mathematical method best suited in each case.
In order to carry out the calculation of the process model, it is provided that the reduction shaft is discretized and the equations of the balances are solved by numerical methods, such as for example the method of finite differences, the method of finite elements, the method of finite volumes or the method of weighted residues. As a result, a numerical method that is sufficiently accurate and appropriate for each balance is available. At the same time, the resolution of the process model may be obtained by successive iteration of the calculation of the balances.
It is further provided that some of the chemical and/or physical properties of the substances involved in the process, in particular for the modelling of the chemical kinetics, the dust deposition and the behaviour of the solid matter, such as internal friction angles and cohesion, are determined from material tests. This ensures that material parameters taken into account in the process model coincide with the actual properties of the materials used.
A further development of the invention is that, to reduce the computing time by utilizing symmetries, the calculation of the process model is performed only for a three-dimensional subregion of the reduction shaft.
It is further provided that the geometry of the reduction shaft is taken over by data transfer from a CAD program. As a result, a change of the geometry of the reduction shaft can be made in the process model with little effort.
It is also advantageous if the results of the calculations are displayed graphically, for example as a sectional representation on a computer screen. As a result, for example, the result of the evaluation of the process model is made available in a clearly presented and quickly comprehensible form as the basis for further measures on the reduction shaft.
One application of the method according to the invention is that a suitable design of the reduction shaft is determined by repeated offline calculation of the process model with variation of the geometry of the reduction shaft. In the design of new reduction plants or the changing of existing reduction plants, an optimum geometry of the reduction shaft is determined in this way, whereby the operating capacity is increased in comparison with a non-optimized geometry. As a result, the geometry of the reduction shaft can be adapted for specific charge materials, for example for the use of iron ores in lump form. Furthermore, the influence of a change of geometry on the reduction process can be investigated in general. On the basis of the results of a repeated calculation of the process model, the geometry necessary for improved process control can be determined.
A further possible application is that the reduction process is optimized by repeated offline calculation of the process model with variation of the process parameters and/or specific charge materials and/or the boundary conditions. As a result, the effect on the reduction process of changes of the feedstock, changes in the raw material composition and changes of other process parameters are investigated. On the basis of the results of the repeated calculation of the process model, the process parameters or charge materials necessary for optimum process control can be determined.
It is further provided that the reduction process is controlled or optimized by online calculation of the process model with the current process parameters taken into account. This makes it possible, for example, for the operator in the control centre of the reduction plant to have better overall control of the reduction process.
The reduction process is optimized by maximizing the degree of metallization or achieving a prescribed degree of metallization of the reduced product with minimal consumption of raw material and/or energy. The evaluations of the process model carried out are to be assessed from this viewpoint.